This invention relates to an analog to digital converter.
A dual slope type analog to digital (A/D) converter with an integrator circuit is now being extensively used because highly accurate digital output values are obtained. In such an A/D converter, a reference voltage and analog input voltage are selectively input to the integrator circuit and the output of the integrator circuit is supplied to a zero cross comparator. The analog voltage is integrated over a predetermined time interval and then the integrated output of the integrator circuit is discharged by connecting the reference voltage to the input terminal. During the time period in which the integrated output reaches a 0 level, i.e., an initial value, clock pulses are counted and a count value is produced as an output digital value. The higher the frequency of the clock pulse becomes, the more the A/D converter makes the output value theoretically higher in accuracy. In this connection it should be noted that an operation lag of the zero cross comparator may be a cause for errors. Moreover, the A/D converter cannot assure a high-speed operation because a discharge period is involved during the operation of the A/D conversion.